In U.S. Pat. No. 3,868,956 there is described a device which after insertion into for example blood vessel may be expanded. The active part of this device is based on the use of metal alloys having so-called "memory function", i.e. a material which when heated will recover its initial configuration. In this prior art the heating of the material is provided by electrical heating, the device being inserted at the location of interest. However, this known technique possesses the essential disadvantage that electrical resistance heating must take place in connection to surrounding sensitive tissue which may be damaged in the heating. It is true that it is stated in the patent specification (cf. col. 3, lines 42-48) that when inserting the device into a blood vessel the patient's blood functions as a cooling medium. However, also blood is a heat-sensitive material which when heated can be subjected to undesirable coagulation.